Separation of mixtures
3 marks · NormalA student has a mixture containing 20.0 g of potassium nitrate (which is soluble in water) and 8.0 g of sand (which is insoluble in water).
She adds 100 cm³ of water and stirs the mixture, dissolving all of the potassium nitrate but none of the sand.
She filters the mixture to separate the sand from the solution and then uses crystallisation to obtain solid potassium nitrate from the filtrate.
Calculate the mass of residue and filtrate obtained after filtration. State which substances are present in each.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
Must show: Residue: 8.0 g sand, Filtrate: potassium nitrate in solution (20.0 g dissolved) + 100 cm³ water, Explanations for which substance is in which portion
Also accept: Residue contains only sand; filtrate contains all potassium nitrate dissolved in water
Common mistakes: Including potassium nitrate in the residue; Omitting the water in the filtrate
Show mark scheme and worked solution
Marking points
M1 · 1 mark · States or calculates that all sand (8.0 g) is residue and all potassium nitrate is dissolved.
A1 · 1 mark · Correctly identifies and gives mass of residue as 8.0 g of sand.
A2 · 1 mark · Correctly identifies filtrate as 20.0 g potassium nitrate dissolved in 100 cm³ water (without sand), or equivalent description.
Worked solution
Potassium nitrate is soluble in water, so all 20.0 g dissolves. Sand is insoluble, so it stays as a solid.
After filtration, the residue is the sand (all 8.0 g) left on the filter paper.
The filtrate is the solution containing all 20.0 g of potassium nitrate dissolved in 100 cm³ water.
Residue: 8.0 g of sand.
Filtrate: 20.0 g of potassium nitrate dissolved in 100 cm³ of water.